The present invention relates to an electronically commutatable motor, whose excitation windings are controllable via semiconductor output stages by an electronic control unit with the aid of PWM control signals, a setpoint value being specifiable to the control unit, and the control unit emitting corresponding PWM control signals to the semiconductor output stages; and a monitoring device for the speed of the motor is provided with which a faulty movement or a standstill of the motor is detectable, and the motor is able to be switched off.
U.S. Pat. No. 5,349,276 describes an electronically, commutatable motor of this type. In this known motor, the speed (=rpm) is predefined and monitored within a specific framework of minimum to maximum rpm. If this framework is left, the engine is switched off. Thus, an immediate and quick detection of the proper revolution, and specifically as a function of a predefined setpoint value of the rpm, is not possible. If difficultiesxe2x80x94sluggishness or locking of the motorxe2x80x94occur during the start-up, the motor can become overloaded and damaged before it gets within the monitored speed range.
Such motors are used for various application cases. Depending upon the type of construction, these motors have a plurality of stop locations which frequently bring with them difficulties for the start-up of the motor, and can result in the motor not starting up, even in response to the input of a setpoint value. In addition, mechanical faults may exist that result in a locking of the motor at standstill, which can lead to overloading upon application of a setpoint value at the control unit.
U.S. Pat. No. 5,682,334 deals with a start-up monitoring of the motor, which, however, does not work with predefined fixed values, but rather has stored a predetermined acceleration characteristic that is defined by points of time and speed. The monitoring is carried out in the manner that a reached motor speed =rpm and the time elapsed since the beginning of the start-up are compared to the time allocated for this speed in the stored characteristic. In this context, the same start-up conditions and a defined preset start-up characteristic are assumed; however, in many cases the motor is put into operation with different start-up conditions for different operating states. This is not easily possible using the known start-up monitoring.
The object of the present invention is to provide an electronically commutatable motor of the type mentioned at the outset, in which monitoring is already carried out at the beginning of the start-up in order to avoid overloading and damage due to overloading.
This objective is achieved according to the present invention in that, when the motor is put into operation by input of a setpoint value and/or switching on the control unit and/or the semiconductor output stages, a start-up time is specifiable to the monitoring device, the start-up time being a function of the magnitude of the predefined setpoint value and preferably increasing as the magnitude of the setpoint value increases, and after the predefined start-up time has elapsed, the monitoring device compares the speed reached by the motor to a minimum speed, and if the minimum speed is not reached, switches off the control unit and/or the semiconductor output stages.
Decisive is a start-up time Tx which is a function of setpoint value Nsetpoint predefined for the start-up, and therefore takes into account the different start-up characteristic curves. Only after this time is it determined whether achieved speed N of the motor has reached a predefined minimum speed Nmin. By appropriate selection of the predefined start-up times Tx, comparison can continually be made to the same minimum speed Nmin. The monitoring expenditure necessary for this is minimal.
According to one embodiment, the control unit and/or the semiconductor output stages can be switched on and off simply by switching the supply voltage on and off.
In addition, the switch-off of the control unit and/or the semiconductor output stages, and thus of the motor, may be time-delayed.
According to a further embodiment, the missing and/or faulty start-up is able to be indicated visually and/or acoustically.
If the monitoring unit and/or the display device is/are integrated into the control unit, then the structural design of the motor does not change, since the electronic control unit also takes over the functions of the monitoring device.